Radiant energy detection systems for use in the visible and infrared portions of the spectrum typically utilize a large number of individual radiation-sensitive elements arrayed in a rectangular grid with the individual elements of the array being "read" or scanned in raster fashion, each picture element ("pixel") within the grid becoming the output pixel in predetermined sequence. Many such systems employ spatial filtering or background normalization for enhancing the capability of the system to detect point sources of radiant energy in the presence of clutter, interference or other background noise. As commonly implemented such filters and normalizers require the sensing not only of the signal from the output pixel itself, but also signals from a number of neighboring pixels the readings from which then are combined in known manner with each other and with the output pixel to correct for background noise as measured at these additional pixel locations.
Such correction signal generally is derived from readings taken from the pixels of a multipixel moving "window" or "mask" which is centered over the output pixel itself and scans with it. Typically such mask may comprise a 3.times.3 square matrix of pixels for spatial filtering applications and a 5.times.5 matrix for background normalizers. The signal as read from the output pixel then is weighted, in the case of spatial filters, or thresholded in the case of background normalizers, in accordance with a signal derived from the signals read from the other pixels comprising the mask. These are themselves weighted and combined in a manner appropriate to the filter or normalizing function being implemented.
Problems arise when the pixel mask is attempted to be scanned to the detector elements situated in the outermost rows and columns of the array, because when the output pixel at the mask center has reached such peripheral row or column, the pixels on the lead or forward side of the mask have run off the adjacent edge of the array or "wrapped around" to the start or end of the next row or column in the raster pattern being scanned. A like problem exists at the initialization of each scan line, when beginning the scan of each row and column, where only some part of the mask will be positioned over the array and some part will lie outside its edges.
To avoid the degradation of accuracy of spatial filtering or normalizing which results from these initialization and wrap-around phenomena, it is known to start and terminate each horizontal and vertical scan at a point one or more pixels removed from the ends of each row and column, so as to make the full scan window remain always within the periphery of the array. By thus discarding or neglecting the outer pixel outputs the problems of initialization and wrap-around are avoided, but there is a price which must be paid. The effective area of the array is substantially reduced; in the particular case of the 5.times.5 mask for a 32.times.32 array background normalizer as described above, for example, such reduction is more than 23%. This effective reduction of array size results in a corresponding reduction of the available field of view of the system and of its performance capability.
The processor of the present invention may incorporate either or both a spatial filtering and a background normalizer function with no reduction of available field of view, and without substantial compromise of performance of the filter and normalizer functions even at the peripheral edges of the array. Further, these objectives are achieved with little if any increase in complexity or cost of the processor.
Therefore, it is a primary object of the present invention to provide a signal processor for a signal detection system which avoids the initialization and wrap-around effects associated with implementation of two-dimensional spatial filters and background normalizers for the signal processing functions in video and infrared detection systems. It is another object of the invention to provide a location dependent signal processor that minimizes the impact of initialization and wrap-around effects on spatial filter weighting and background normalizer thresholding while avoiding reduction of available field-of-view.